1. Field of the Invention
The present invention relates to a thermistor having a positive coefficient (hereinafter referred to as PTC (positive temperature coefficient) characteristic) where electric resistance remarkably increases with an increase in temperature, particularly relates to a thin-film thermistor having the PTC characteristic, and more particularly relates to a PTC thin-film thermistor obtained by utilizing a barium titanate based composition.
2. Description of the Related Art
A PTC characteristic has conventionally been known in bulk materials of barium titanate based semiconductor ceramics obtained by adding rare earth elements such as Y and La to bulk barium titanate and burning the mixture in the air at 1200.degree.-1400.degree. C. Heaters and temperature sensors have been prepared by utilizing the characteristic. The maximum resistance variation rate has been at most about 0.1 order/.degree.C. and has been very unsatisfactory. The temperature where electric resistance increases can be shifted to a low temperature side or a high temperature side by replacing a portion of the Ba sites in said ceramic materials with Sr or Pb, respectively. Thus said temperature can be arbitrarily changed in the range of from -30.degree. C. to 300.degree. C.
However, according to the information of the inventors, conventional PTC thermistors have been very small in maximum resistance variation rate and prepared by mixing and burning oxide of each constituting element such as Ti and Ba in prescribed concentration. Consequently, these thermistors inevitably have a large thickness and also result in large resistance at room temperature. These problems must be overcome by enlarging the area of electric circuit to reduce resistance.